Starting with magnetic phenomena on Earth (e.g., Fig. 1), a brief summary of our observational knowledge of the magnetic fields has been presented for the interplanetary medium (Fig. 5), for objects therein (Figs. 2, 3), and their association with angular momentum (Fig. 4). The methodology of signal detection from Earth has been discussed (Figs. 7, 8). Remanent magnetism (random shape) or dynamo magnetism (dipolar shape) can explain the maintenance of magnetism in most solar system objects. In a few cases, a dead or dying dynamo may be involved. The interplanetary medium supports an Archimedean spiral-shaped magnetic field, usually with 4 magnetic sectors (e.g., Fig. 6).
Outside the solar system, the magnetism of normal stars can be mostly explained by a large scale dipolar dynamo, with the possible addition of small scale loop-type magnetism localized on the stellar surfaces. In a few cases, a quadrupolar shape field is necessary. For compact degenerate stars, a frozen dipolar magnetic field is found.
Over recent years, direct measurements of the magnetic field strength and direction in dusty protostellar disks and dusty molecular cloudlets have shown that hydromagnetic processes are essential to understanding their internal physics and evolution. The ultimate goal of obtaining a full understanding of the magnetic field in the formation and evolution of molecular disks and cloudlets is a challenging task. Is the magnetic field a strong player/guide or else a weak player/tracer ? A preliminary answer favours a moderately strong (but not dominant) magnetic field.
Observational studies have been mainly limited by numerous practical difficulties of measuring magnetic fields in astronomical objects, via dust emission (weighted by grain properties), or via Faraday rotation (weighted by electron density) or via Zeeman effect (with instrumental sensitivity effects) (e.g., Davies 1994).
A succinct review and a short classification of various magnetic field models for protostellar disks has been made (Figs. 10 to 14), and a time evolution discussed (Fig. 17). Although the geometry of the magnetic field could be partially preserved (e.g., poloidal) when clouds contract from the interstellar medium (Fig. 21; e.g., Jones et al. 1992), after a while the increasing differential rotation of the protostellar disk may completely change the B lines to become spiral (toroidal). Cool protostellar disks and cloudlets are only detected in emission at Extreme IR and Far IR wavelengths, and their low polarized flux density values present a technical challenge for such telescopes (e.g., Fig. 9).
Magnetic protostellar models involve "twist", "circulation", "cloud collapse", "disk-wind", "magnetic pinch", "hourglass", as well as "dynamo".
A list of observational data for magnetism in cloudlets/protostellar disks has been presented (Table 1). Possible correlations have been discussed: polarization percentage correlating with wavelength, with beam size, with source age, and polarization position angle correlating with companion presence (Fig. 19), with viewing angle, with beam size, and with source age. For disks/cloudlets, the main predictions (Fig. 15, 16) and the effect of telescope beams (Fig. 18) have been noted and used in comparing selected observational maps (Figs. 20, 22, 23).
Future trend: polarimetry at Extreme IR and Far IR is the most effective way to determine unambiguously the direction (position angle) of the magnetic field in protostellar disks and in molecular cloudlets, since it measures the emission of polarized radiation. Scattered light is not a problem at long wavelengths. Extreme-IR (submillimeter), Far IR, and mid-IR wavelength polarization observations are not affected by dust scattering since light scattering varies as (wavelength)-4 .
Clearly this research area at Extreme-infrared wavelengths is in its infancy, much in need of more polarimetric data, and has a potential for a rapid growth in our physical understanding. The time is ripe for a major observational advance, with the recent improvements in polarimetric technology in the Extreme IR and Far IR. At these wavelengths, polarimetry is a powerful method to study circumstellar, protostellar, and interstellar magnetic fields. and it is limited by the Earth's atmosphere, observing time required, and lack of polarimetric instruments at many telescopes.
A good polarimetric map in the Extreme IR and Far IR will in itself say a good deal, and it may inspire new theoretical work. Magnetic field orientations are crucial in most models of formation and evolution of disks and cloudlets and in star formation. Sensitive array receivers could open up a rich and exciting field of study in polarimetry. A reasonable 10 × improvement in sensitivity would result in a decrease in observing time by a factor 100, when aiming for the same signal to noise ratio.
I thank Ms. Lyne Séguin (NRCC-Ottawa) for creative help in drawing Figures 10 to 14 and 16, Mr. David Duncan (NRCC-Victoria) for drawing Figures 1, 2, 3, 5, 7, 8, 17, while I used the PGPLOT software for Figures 4 and 19. I thank a referee (anonymous) for thoughtful and valuable advice, and Dr. D. C. Morton (NRCC-Victoria) for a reading of an early version.